This invention relates to a heat shrinkable, coextruded polyethylene film laminate suitable for the heat-shrink packaging of an article.
A heat-sealable, biaxially oriented polyethylene film laminate is known which includes a core layer of a relatively higher melting point polyethylene between two surface layer of a relatively low melting point polyethylene. A heat-resisting, biaxially oriented polyethylene film laminate is also known which includes a core layer of a relatively low melting point polyethylene between two surface layer of a relatively high melting point polyethylene. Because of poor stretchability of polyethylene in comparison with polypropylene resin, however, it is difficult to prepare the above polyethylene film laminates. In particular, the temperature range in which the stretching of polyethylene films can be carried out smoothly in a satisfactory manner is so narrow that it is difficult to continue stretching of the polyethylene films in a stable manner for a long time. As a consequence, known oriented polyethylene film laminates have defects that the thickness thereof is not uniform and that stretching cannot be continued in a stable manner for a long process run.
It is, therefore, an object of the present invention to provide a biaxially oriented, coextruded polyethylene film laminate which has uniform thickness.
Another object of the present invention is to provide a biaxially oriented, coextruded polyethylene film laminate which has good stretchability in both high and low temperatures and which can be continuously stretched in a stable manner even when the stretching temperature fluctuates.
It is another object of the present invention to provide a biaxially oriented, coextruded polyethylene film laminate of the above-mentioned type which shows good heat resistance and good surface slippage (good wrapping or packaging property).
In accomplishing the foregoing objects, there is provided in accordance with the present invention a polyethylene laminate film comprising two surface layers F1 and F2, and an intermediate layer M interposed therebetween,
wherein the surface layer F1 comprises
a linear low density polyethylene having a density DAF1 g/cm3 of 0.910-0.930 g/cm3 and a linear high density polyethylene having a density DCF1 g/cm3 of 0.925-0.945 g/cm3 and being present in an amount of WCF1 % based on a total weight of the linear low density polyethylene having the density DAF1 and the linear high density polyethylene having the density DCF1,
wherein the surface layer F2 comprises
a linear low density polyethylene having a density DAF2 g/cm3 of 0.910-0.930 g/cm3 and
a linear high density polyethylene having a density DCF2 g/cm3 of 0.925-0.945 g/cm3 and being present in an amount of WCF2 % based on a total weight of the linear low density polyethylene having the density DAF2 and the linear high density polyethylene having the density DCF2,
wherein the intermediate layer M comprises
a linear low density polyethylene having a density DAM g/cm3 of 0.910-0.930 g/cm3 and
a linear very low density polyethylene having a density DBM g/cm3 of 0.880-0.915 g/cm3 and being present in an amount of WBM % based on a total weight of the linear low density polyethylene having the density DAM and the linear very low density polyethylene having the density DBM, and
wherein the laminate film satisfies the following conditions (a) through (e) at the same time:
(a) DCF1xe2x88x92DAF1xe2x89xa70.010,
(b) DCF2xe2x88x92DAF2xe2x89xa70.010,
(c) DAMxe2x88x92DBMxe2x89xa70.010,
(d) 0.01xe2x89xa6{(DCF1xe2x88x92DAF1)xc3x97LF1xc3x97WCF1}+{(DCF2xe2x88x92DAF2)xc3x97LF2xc3x97WCF2}xe2x89xa60.20,
(e) 0.05xe2x89xa6(DAMxe2x88x92DBM)xc3x97LMxc3x97WBM}xe2x89xa61.40,
xe2x80x83where DCF1, DAF1, DCF2, DAF2, DAM, DBM, WCF1, WCF2 and WBM are as defined above and LF1, LF2 and LM represent as follows:
LF1=tF1/(tF1+tF2+tM),
LF2=tF2/(tF1+tF2+tM),
LM=tM/(tF1+tF2+tM),
where tF1, tF2 and tM represent the thicknesses of the surface layer F1, surface layer F2 and intermediate layer M, respectively.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.
In the polyethylene film laminate according to the present invention, a linear low density polyethylene (hereinafter referred to as LLDPE) having a density of 0.910-0.930 g/cm3 is used in each of surface layers F1 and F2 and in an intermediate layer M interposed therebetween. It is preferred that LLDPE have a melt index of 0.5-5.9 g/10 min for reasons of good stretchability. LLDPE is preferably a copolymer of ethylene with an xcex1-olefin having 4-8 carbon atoms. Copolymers produced using a Ziegler Natta catalyst or a single site catalyst may be used for the purpose of the present invention.
In addition to LLDPE, each of the surface layers F1 and F2 contains linear high density polyethylene (hereinafter referred to as LHDPE) having a density of 0.925-0.945 g/cm3. The intermediate layer M must contain linear very low density polyethylene (hereinafter referred to as VLDPE) having a density of 0.880-0.910 g/cm3 in addition to LLDPE.
It is preferred that LVLDPE have a melt index similar to LLDPE and in the range of 0.5-5.0 g/10 min for reasons of good streatchability and of compatibility with LLDPE. LVLDPE is preferably a copolymer of ethylene with an xcex1-olefin having 4-8 carbon atoms. Copolymers produced using a Ziegler Natta catalyst or a single site catalyst may be used for the purpose of the present invention.
It is preferred that LHDPE have a melt index similar to LLDPE and in the range of 0.5-5.0 g/10 min for reasons of good streatchability and of compatibility with LLDPE. LHDPE is preferably a copolymer of ethylene with an xcex1-olefin having 4-8 carbon atoms. Copolymers produced using a Ziegler Natta catalyst or a single site catalyst may be used for the purpose of the present invention.
In the present invention, it is not necessary that the same polyethylene be used in the surface and intermediate layers of the same laminate. Thus, different LLDPEs can used in respective layers of the same laminate. Similarly, respective layers of the same laminate can contain different LVLDPEs and different LHDPES.
The surface layers F1 and F2 and the intermediate layer M of the laminate according to the present invention must meet with specific conditions. These conditions will be next described.
As described above, the surface layers F1 and F2 contain LHDPE in addition to LLDPE, while the intermediate layer M contains LVLDPE in addition to LLDPE. In this case, it is important that the following conditions (a) through (c) are met at the same time:
(a) DCF1xe2x88x92DAF1xe2x89xa70.010,
(b) DCF2xe2x88x92DAF2xe2x89xa70.010,
(c) DAMxe2x88x92DBMxe2x89xa70.010,
xe2x80x83wherein
DCF1: density (g/cm3) of LHDPE of surface layer F1
DAF1: density (g/cm3) of LLDPE of surface layer F1
DCF2: density (g/cm3) of LHDPE of surface layer F2
DAF2: density (g/cm3) of LLDPE of surface layer F2
DBM: density (g/cm3) of LVLDPE of intermediate layer M
DAM: density (g/cm3) of LLDPE of intermediate layer M.
Namely, in each of the surface layers F1 and F2, the density of LHDPE must be greater by at least 0.010 g/cm3 than that of LLDPE, in order to obtain good stretchability and heat resistance. At the same time, in the intermediate layer M, the density of LLDPE must be greater by at least 0.010 g/cm3 than that of LVLDPE in order to obtain good stretchability.
Further, it is important that the following conditions (d) and (e) should be met simultaneously:
(d)
0.01xe2x89xa6ZS1+ZS2xe2x89xa60.20,
ZS1=(DCF1xe2x88x92DAF1)xc3x97LF1xc3x97WCF1 
ZS2=(DCF2xe2x88x92DAF2)xc3x97LF2xc3x97WCF2 
(e)
0.05xe2x89xa6ZMxe2x89xa61.40
ZM=(DAMxe2x88x92DBM)xc3x97LMxc3x97WBM 
xe2x80x83where DAF1, DCF1, DAF2, DCF2, DBM and DAM are as defined above, WCF1, WCF2 and WBM represent as follows:
WCF1: amount of LHDPE (% by weight) based on a total weight of LLDPE and LHDPE of the surface layer F1,
WCF2: amount of LHDPE (% by weight) based on a total weight of LLDPE and LHDPE of the surface layer F2,
WBM: amount of LVLDPE (% by weight) based on a total weight of LLDPE and LVLDPE of the intermediate layer M,
xe2x80x83and LF1, LF2 and LM represent as follows:
LF1=tF1/(tF1+tF2+tM),
LF2=tF2/(tF1+tF2+tM),
LM=tM/(tF1+tF2+tM),
where tF1, tF2 and tM represent the thicknesses of the surface layer F1, surface layer F2 and intermediate layer M, respectively.
When (ZS1+ZS2) is smaller than 0.01, the range of temperature in which the laminate can be stretched in a stable manner at a high temperature is so small that it is difficult to stretch the laminate at such a high temperature for a long period of time. Further, the laminate becomes poor in heat resistance. When (ZS1+ZS2) is greater than 0.20, on the other hand, it is possible to stretch the laminate at a high temperature. However, the strechability becomes no good and the thickness of the stretched film becomes non-uniform. Very good stretchability and surface slippage (wrapping property) are obtainable when (ZS1+ZS2) is in the range of 0.04 to 0.12 and this range represents the preferred range.
When ZM is smaller than 0.05, the range of temperature in which the laminate can be stretched in a stable manner at a low temperature is so small that it is difficult to stretch the laminate at such a low temperature. When ZM is greater than 1.4, on the other hand, it is not possible to stretch the laminate at a low temperature in a stable manner. When a hard package is desired, it is preferred that ZM be in the range of 0.10 to 0.30. On the other hand, when a soft package is desired, such as for wrapping sheets or cards, it is preferred that ZM be in the range of 0.70 to 1.20.
If desired, one or more layers may be interposed between the surface layers S1 and S2. For example, a layer of a resin obtained from recycled laminates or noncompliant laminates may be incorporated into the laminate of the present invention as an additional intermediate layer. In such a case, the additional layer may be sandwiched between two intermediate layers M, with surface layers S1 and S2 being provided on opposite sides to form a five-layered laminate. The laminate generally has a thickness of 8-50 xcexcm, preferably 10-35 xcexcm. The thickness of each of the surface layers S1 and S2 is generally 5-33, preferably 10-25% of the total thickness.
Each of the surface layers S1 and S2 and the intermediate layer M may contain one or more conventional additives such as an antioxidizing agent, an antistatic agent, a lubricant, an antiblocking agent, a colorant and a filler. A crosslinking agent, a crosslinking accelerator, etc, may also be incorporated into the laminate to crosslink the laminate before or after stretching. Further, a resin such as a high pressure low density polyethylene having a density of 0.91-0.93 may be used in conjunction with LLDPE, LHDPE and/or LVLDPE in an amount of less than 15% by weight based on the laminate.
The laminate of the present invention may be prepared by any suitable conventional method such as coextrusion. The extruded sheet is stretched, preferably biaxially, by tentering or inflation method to obtain a heat-shrinkable, coextruded polyethylene film laminate suitable for the heat-shrink packaging of an article. Packages obtained by using the heat-shrinkable laminate of the present invention do not melt or whiten during their passage through a heat-shrinking tunnel. Further, the heat-shrunken films of the packages are free of non-shrunken portions and are maintained in a tense state.
The following examples will further illustrate the present invention. Parts and percentages are by weight.